1. Field of the Invention
The present invention relates to a semiconductor substrate having a hetero-epitaxial layer, a solar cell using this semiconductor substrate, and methods of fabricating the semiconductor substrate and solar cell.
2. Description of the Related Art
Hetero-structure semiconductor devices are being studied for use as high-performance solar cells in spacecraft, to mention just one application. The goal of current research in this area is to develop solar cells with very high conversion efficiency, combined with light weight, high mechanical strength, and low cost. One promising approach grows compound semiconductor layers epitaxially on, for example, a silicon substrate. Silicon provides the requisite high mechanical strength, and is lightweight and low in cost. The compound semiconductor layers provide a higher conversion efficiency than is obtainable from silicon alone.
One compound semiconductor material that provides particularly high conversion efficiency is indium gallium phosphide (InGaP). To grow InGaP on a silicon substrate, however, it is first necessary to add a buffer layer such as a hetero-epitaxial layer of gallium arsenide (GaAs) to the substrate, forming a gallium-arsenide/silicon (GaAs/Si) semiconductor substrate.
A basic problem in the fabrication of this substrate is that of growing a high-quality GaAs hetero-epitaxial layer on a silicon substrate layer. This problem was addressed by M. Akiyama in a thesis published in 1991. His solution was a three-step process comprising a high-temperature surface-cleansing step followed by two epitaxial growth steps, the first growth step being performed at a comparatively low temperature. In the high-temperature surface-cleansing step, arsine gas is used to prevent surface contamination, and hydrogen is used as a carrier gas.
This thesis also reported that it was advantageous to use a silicon substrate cut from a silicon crystal ingot at an angle of, for example, 0.5xc2x0 to 15xc2x0 with respect to the (100) crystal lattice plane. The GaAs hetero-epitaxial layer acquires a similar inclination. This inclination is oriented in, for example, the  less than 011 greater than  direction of the silicon crystal lattice, and the  less than 0-11 greater than  direction of the GaAs crystal lattice. The reason for this particular combination of orientations is that during epitaxial growth, arsenic atoms bind first to the silicon surface.
When solar cells are fabricated by growing InGaP layers on a semiconductor substrate with a GaAs buffer layer formed in this way, however, the conversion efficiency is not as high as expected. It is known that the  less than 0-11 greater than  inclination of the GaAs hetero-epitaxial layer promotes an ordering of the InGaP layers, and this is thought to be one reason for the unexpectedly low conversion efficiency, the ordering leading to reduced carrier mobility. It is also known that the ordering of the InGaP layer can be suppressed if the inclination of the GaAs buffer layer is aligned in the  less than 011 greater than  direction of the GaAs crystal lattice, which is oriented at right angles to the  less than 0-11 greater than  direction, and it is suggested in the above-mentioned thesis that the desired alignment might be achieved if pure hydrogen gas, instead of a mixture of arsine and hydrogen, were to be used in the initial high-temperature surface cleansing step. Unfortunately, hydrogen gas by itself is incapable of preventing surface contamination of the silicon substrate, leading to poorly formed GaAs layers.
An alternative method of preventing contamination is to clean the epitaxial growth apparatus each time it is used, but this method is unattractive because it lowers production efficiency and raises fabrication costs.
An object of the present invention is to provide a semiconductor substrate having a group-IV semiconductor layer and a group-III-V compound semiconductor hetero-epitaxial layer, in which the surface of the group-III-V compound semiconductor hetero-epitaxial layer is of high quality and is inclined in the  less than 011 greater than  crystal lattice direction.
Another object of the invention is to provide a solar cell with improved conversion efficiency.
The invented semiconductor substrate comprises a group-IV semiconductor substrate layer and a group-III-V compound semiconductor hetero-epitaxial layer. The group-IV semiconductor substrate layer is cut from a group-IV semiconductor ingot at an angle such that its front surface is inclined with respect to the (100) crystal lattice plane in a direction differing from the  less than 010 greater than  crystal lattice direction. The group-III-V compound semiconductor hetero-epitaxial layer is grown on the inclined front surface of the group-IV semiconductor substrate layer. The  less than 011 greater than  crystal lattice direction of the group-III-V compound semiconductor hetero-epitaxial layer is substantially aligned with the direction of inclination of the front surface.
The group-IV semiconductor substrate layer comprises, for example, silicon or germanium. The group-III-V compound semiconductor hetero-epitaxial layer comprises, for example, gallium arsenide or indium phosphide.
The invented fabrication method for the invented semiconductor substrate includes the steps of:
(a) obtaining a group-IV semiconductor substrate layer cut from an ingot at an angle as described above;
(b) heating the substrate in the presence of a gas including a compound of a group-IV element present in the substrate layer, to clean the substrate layer;
(c) supplying a source gas of a group-III element, thereby forming an atomic film of the group-III element on the surface of the substrate layer; and
(d) supplying a source gas of a group-V element together with the source gas of the group-III element, thereby growing a group-III-V compound semiconductor hetero-epitaxial layer on the substrate layer.
The gas used in step (b) is preferably a hydrogen or halogen compound of the group-IV element. The use of this gas for cleansing permits atoms of the group-III element to bind chemically with the substrate surface in step (c), which in turn produces the desired alignment of the crystal lattice directions of the hetero-epitaxial layer. This cleansing process also effectively protects the surface of the group-IV substrate layer from contamination, producing a high surface quality that carries over to the surface of the group-III-V compound semiconductor hetero-epitaxial layer.
Supply of the two source gases used in steps (c) and (d) may begin simultaneously, or the supply of the source gas for the group-V element may begin after the source gas for the group-III element is already being supplied.
The invented solar cell comprises the invented semiconductor substrate, a compound semiconductor base layer, and a compound semiconductor emitter layer. The compound semiconductor base layer and emitter layer comprise, for example, indium gallium phosphide.
The invented method of fabricating a solar cell comprises the steps (a) to (d) described above, followed by the formation of the base layer and the emitter layer.
The alignment of the  less than 011 greater than  crystal lattice direction of the group-III-V compound semiconductor hetero-epitaxial layer in the-general direction of surface inclination reduces ordering in the compound semiconductor base layer and emitter layer, thereby enhancing the conversion efficiency of the solar cell.